High-Energy Plasma Generator Using Radio-Frequency and Neutral Beam Power

ABSTRACT

An apparatus for generating a highly energetic plasma employs a low-energy neutral beam injected into a magnetically contained mirror plasma to produce plasma ions boosted in energy to fusion levels by a coordinated radiofrequency field.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT

This invention was made with government support under DE-SC0002322awarded by the US Department of Energy. The government has certainrights in the invention.

CROSS REFERENCE TO RELATED APPLICATION

This application claims the benefit of U.S. patent application Ser. No.16/839,780 filed Apr. 3, 2020 of which is hereby incorporated byreference.

BACKGROUND OF THE INVENTION

The present invention relates to an apparatus for generating high-energyplasmas that can promote nuclear fusion, and in particular, to systemsusing magnetic mirror confinement and neutral beam injection, withadditional radiofrequency power injection.

High-temperature plasmas can be confined away from a physical containerand avoiding damage to the container and possible plasma quenching, by amagnetic mirror confinement system. Such confinement systems may providean axial magnetic field extending between two ends at which the magneticflux lines converge. Plasma ions moving within this axial magnetic fieldspiral along the flux lines at the local cyclotron frequency and are“reflected” by an axial component of magnetic force acting on thespiraling ions. This reflecting magnetic force caused by the flux lineconvergence and concomitant increasing magnetic field strength is in thedirection away from the convergence. Moreover, the reflecting three isproportional to the particle kinetic energy component which isperpendicular to the magnetic field, A similar reflecting force acts onthe plasma electrons.

Nuclear fusion can be promoted in a magnetic mirror confinement systemby generating plasma with sufficiently high energy and density. Onemethod of reaching this high-energy/density state injects electricallyneutral particles (a neutral beam) through the magnetic containmentfield into the plasma where the neutral particles of the neutral beamare ionized, that is, split into plasma ions and electrons. The neutralbeam has an initial energy above that necessary for fusion so that theresulting plasma ions maintain an energy suitable for fusion even withan expected collisional loss of energy of the plasma ions afterintroduction into the plasma. The plasma density and energy aredetermined by the loss rate of the fast ions injected by neutral beamswhich decreases with increasing beam energy; hence high energy ions arebetter confined than low energy ions.

A neutral beam generating a sufficient flux of highly energeticparticles at energies sufficient to maintain high fusion output in amagnetic mirror confinement system is difficult and costly from anenergy standpoint. Currently, such an approach does not appear to bepractical for net fusion energy generation.

SUMMARY OF THE INVENTION

The present invention also injects a neutral beam injection into amagnetic mirror confinement but differs from the previous approaches byemploying a low-energy neutral beam having far less energy than neededto produce significant fusion directly. Instead, after the neutral beamis ionized, the energy of those neutral beam sourced fast ions isboosted within the magnetic containment volume by using a radiofrequencyelectrical field. The difficulties of preferentially transferringradiofrequency energy to the fast-neutral beam ions rather than thermalions is overcome by controlling the injection angle and energy of theneutral beam so that there is a well-defined “turning point” of the fastions in the magnetic containment field. Tuning the radiofrequency wavesto a multiple (i.e. a harmonic) of the cyclotron frequency at theturning point, preferentially energizes these neutral beam injected ionsto fusion levels with only small expected wave damping effects onthermal ions.

Specifically, then, in one embodiment, the invention provides anapparatus for producing high-energy plasma in a magnetic mirrorcontainment field, the latter providing axially-extending magnetic fluxlines converging at opposed first and second ends of a containmentvolume holding the plasma. A neutral beam generator directs a neutralbeam of particles into the containment volume at a predetermined pitchangle with respect to the magnetic field and an energy range so that theparticles disassociate into plasma ions at the same pitch angle withinthe containment volume and have a well-defined turning point. At theturning point, fast ions have purely perpendicular energy. Aradiofrequency generator can then be used to produce an t electricalfield to accelerate the beam-sourced ions to an energy sufficient forfusion of the plasma ions.

It is thus a feature of at least one embodiment of the invention toprovide a system for boosting the energy of the plasma ions afterinjection into the containment field, greatly increasing the efficiencyof the neutral beam.

The frequency of the electrical field may be functionally dependent on acyclotron frequency at turning points for the plasma ions of the neutralbeam in the magnetic mirror containment field.

It is thus a feature of at least one embodiment of the invention topreferentially deposit energy in the plasma ions having a matchingcyclotron frequency.

In one embodiment, the frequency of the electrical field may be aharmonic of the cyclotron frequency at the turning point, greater thanthe cyclotron frequency.

It is thus a feature of at least one embodiment of the invention toexploit preferential transfer of radiofrequency electrical energy toresonant fast ions that occurs at higher cyclotron harmonics.

The energy of the neutral beam is set so that more than 50 percent ofthe neutral beam particles are converted to plasma ions.

It is thus a feature of at least one embodiment of the invention topermit the use of a lower energy neutral beam amenable to higherparticle flux and thus capable of high plasma densities.

The neutral beam may have an energy of less than 50,000 electron volts.

It is thus a feature of at least one embodiment of the invention toallow setting the trade-off in the design of the neutral beam generatorfor high flux rates rather than high energies thereby improving ionfueling rates.

The radiofrequency generator may boost the energy of the plasma ionsfrom the neutral beam by more than 2 times.

It is thus a feature of at least one embodiment of the invention toprovide significant energy boosting of the plasma ions after injection.

The radiofrequency generator may include an antenna positioned to beproximate to a reflection limit of the plasma ions and to generate arotating electric vector perpendicular to the axis of the magneticmirror containment field.

It is thus a feature of at least one embodiment of the invention tooptimize the antenna for energy deposition of the plasma ions.

The angle of the neutral beam may be between 15° and 80° to the axis.

It is thus a feature of at least one embodiment of the invention toprovide a good trade-off between energy of the neutral beam and aturn-around point that isolates the neutral beam from thermal ions.

The apparatus may further include a treatment volume at least partiallysurrounding the containment volume to receive high-energy neutronstherethrough and containing an element for transmutation into adifferent element.

It is thus a feature of at least one embodiment of the invention toprovide a system for treatment of materials with neutrons, for example,to create radiopharmaceuticals or to revitalize spent nuclear fuel.

The neutral beam may be selected from the group consisting of deuteriumand tritium and in some embodiments the system may use deuterium onlywith respect to the neutral beam and the gas in the containment volume.

It is thus a feature of at least one embodiment of the invention toprovide a system that can work with well-understood neutral beammaterials and in some cases that can avoid the use of tritium in favorof deuterium.

In one embodiment, the invention may be employed to create a fusionapparatus having a reaction volume holding a fusible material within afirst axially-extending magnetic containment field. In this embodiment,a first and second plasma plug may flank the reaction volume along theaxis, each plasma plug being the apparatus for producing high-energyplasma as described above, wherein plasma ions escaping from the firstand second plasma plugs produce a fusion reaction in the reactionvolume.

It is thus a feature of at least one embodiment of the invention toprovide an improved design for a fusion device for providingtransmutation or power generation.

These particular objects and advantages may apply to only someembodiments falling within the claims and thus do not define the scopeof the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a perspective, cutaway view of a first embodiment of theinvention providing a magnetic mirror containment field, a neutral beamgenerator for directing beams into the containment volume, and aradiofrequency generator producing an electric field acting on plasmaions from the neutron beam generator;

FIG. 2 is a side, elevational view of flux lines of the containmentvolume of FIG. 1 , aligned with an end view of those flux lines, both ofthese views showing trajectories of plasma ions of different energies,as well as graphs of cyclotron frequency, dwell time and electric fieldstrength as a function of axial distance; and

FIG. 3 is a simplified elevational cross-section of a fusion apparatusemploying the magnetic mirror containment fields of FIG. 1 as plugsblocking the escape of high-energy plasma ions from the centralsolenoidal magnetic field cell.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

Referring now to FIG. 1 , a high-energy plasma system 10 may provide apressure vessel 12, for example, in the form of a sealed cylindricalshell of stainless steel or the like, extending along an axis 14 forreceipt of a reaction gas, such as deuterium or tritium, through valveinlet assembly 13 from a pressure tank or the like (not shown).

First and second electromagnetic coils 16 a and 16 may be positionedwithin the pressure vessel 12 near the opposed ends of the pressurevessel 12 to define a containment volume 17 therebetween having amagnetic containment field 15. The electromagnetic coils 16 are orientedand separated to form a Helmholtz pair aligned along axis 14 forestablishing an axial Bo field therebetween. In one embodiment, theelectromagnetic coils 16 may be pancake coils providing spirals aboutaxis 14 powered by an external, controllable DC power supply 18 of thetype understood in the art.

Positioned between the electromagnetic coils 16 but proximate to oneelectromagnetic coil 16 b is a radiofrequency antenna 19 (shown insimplified form), for example, providing a circularly polarized radiofield extending along axis 14 when driven by a radiofrequency generator20. As is understood in the art, the polarized radio field provides anelectrical vector 21 perpendicular to axis 14 rotating thereabout.Further discussion of loop antennas suitable for this purpose are foundin T. H. Stix, “Fast Wave Heating of a Two-Component Plasma,” NuclearFusion 15, 737 (1975) and R. W. Harvey, M. G, McCoy, G. D. Kerbel, andS. C. Chiu, “ICRF Fusion Reactivity Enhancements in Tokamaks,” NuclearFusion 26, 43 (1986) hereby incorporated by reference.

A treatment volume 22 may be located radially outside the pressurevessel 12, for example, in the form of a concentric outer cylindricaltank which may be filled with, for example, an aqueous material fortransmutation by high-energy neutrons such as precursors to medicalisotopes ⁹⁹Mo (molybdenum 99), ¹³¹I (iodine 131), ¹³³Xe (xenon 133) and¹⁷⁷Lu (lutetium 177) or which may support racks holding spent nuclearfuel rods being rejuvenated through transmutation by high-energyneutrons.

A neutral beam generator 26 is positioned to inject a beam 28 of neutralparticles 29 (that is non-ionized particles having zero net charge) at apitch angle θ into the containment volume 17. The pitch angle θ isdefined as an acute angle between an angle of the beam 28 and the axis14. The neutral particles 29, for example, are atoms of deuterium ortritium introduced through a gas line 24 and ionized by a local plasma(not shown). These ions are accelerated in an accelerator chamber 27having a successive set of electrically charged plates as is generallyunderstood in the art. The ions then pass through a neutralizing gascell 31 to produce neutral particles 29 by a charge exchange process toproduce the neutral particles 29 of the beam 28.

Referring now also to FIG. 2 , the magnetic flux lines 30 generated bythe coils 16 will produce a “bottle” shape expanding radially from theaxis 14 at a midpoint between the coils 16 and contracting radially atthe location of the coils 16. As is generally understood in the art,this configuration produces a mirror containment volume where randomlydistributed “thermal” plasma ions of sufficient pitch angle 32 spiralaround flux lines 30 between regions defined by turning points 34.

These thermal plasma ions can be established in a variety of ways forexample by using the radiofrequency antenna 19 (albeit at a lowefficiency) or a separate heating system using high-frequency microwavesproducing electron cyclotron resonance heating, as is understood in theart

At the regions of the turning points 34, the thermal plasma ions 32reverse direction caused by increasing axial components of the magneticLorentz force produced by the convergence of the flux lines 30. Thefrequency 35 of the spiraling about the flux lines 30 is termed the“cyclotron frequency” and is a function of the strength of the magneticfield 37 along axis 14, and for this reason the cyclotron frequency 35generally increases toward the electromagnetic coils 16. For ions ofequal mass and charge, the cyclotron frequencies will be nominallyidentical at a given location along the axis 14, independent of thevelocities or energies of the ions; however, ions 32 of equal masshaving different pitch angles will normally have different turningpoints 34.

The velocity and hence the energy of the neutral particles 29 of theneutral beam 28 and the pitch angle θ of the neutral beam 28 are set sothe majority, for example, greater than 50 percent, of the particles ofthe neutral beam 28 will be ionized into plasma ions 36 within thecontainment volume 17 before exiting the containment field. These plasmaions 36 at the same pitch angle, now having electrical charge, arecaptured by the magnetic flux lines 30 to increase the plasma density.

In order to promote this entrapment of the majority of the neutralparticles 29 of the neutral beam 28, the energy of the neutral beam 28is limited to provide sufficient time-of-flight for the neutralparticles 29 to be ionized. Generally, the desirable energy of theneutral beam 28 for ionization will be well below the kinetic energyrequired for substantial fusion, and typically less than 100 thousandelectron volts or preferably less than 50,000 electron volts and moretypically on the order of 15-25 keV. This is in contrast to prior artapproaches which require neutral particles 29 with energies exceedingthe energy necessary to promote fusion between the plasma ions 36 andtypically having energies more than one million electron volts for D-Dfusion, By limiting the energy of the neutral beam 28, a trade-off maybe affected in common neutron beam generators 26 to produce a higherflux density of neutral particles 29, also increasing the plasmadensity.

Referring still to FIGS. 1 and 2 , the pitch angle θ of the neutral beam28 is selected to provide predetermined turning points 34′ along axis 14for the resulting plasma ions 36 and thus to provide a correspondingpredetermined cyclotron frequency 35 of the plasma ions 36 at theturning points 34′. This cyclotron frequency is used to set thefrequency of the radiofrequency generator 20 as will be discussed below.

In addition, the antenna 19 is placed proximate to one of the turnaroundpoints 34′ to provide a maximum field strength in that region.

Finally, within the energy levels for the neutral beam 28 that providethe desired capture of the neutral particles 29 within the containmentvolume 17, the energy of the neutral beam 28 is set to be as high aspossible so that the radius of orbit of the of the plasma ions 36produced by the neutral beam 28 (gyro-orbit 52) is higher than theaverage distribution gyro-orbit 52 of “thermal ions” 32, being ions notimmediately derived from the neutral beam 28.

While the inventors do not wish to be bound by a particular theory, theabove-described: (a) setting of the cyclotron frequency of theradiofrequency generator 20 to a harmonic of the cyclotron frequency ofthe plasma ions 36 at the turning point 34′, (b) boosting of the energyof the plasma ions 36 above the average distribution of the thermalplasma ions 32, and (c) maximizing the electrical field strength at theturning point 34′, all work together to allow the radiofrequencygenerator 20 to preferentially boost the energy of the plasma ions 36from the neutral beam 28 tree from the damping effect of thermal plasmaions 32.

In this regard, the setting of the radiofrequency generator 20 (per (a))provides preferential coupling to the plasma ions 36 having a matching(e.g., a harmonically related) cyclotron frequency 35, in contrast tothermal plasma ions 32 having a range of different Doppler-shiftedcyclotron frequencies and less effective coupling. The coupling may beproportional to the square of the Bessel functionB_(n-1)(k_(⊥)*ν_(⊥)/ω_(ci)) where:

-   -   n is the resonant cyclotron harmonic number of the injected        wave,    -   k_(⊥) the perpendicular wave number, and    -   ω_(ci) is the cyclotron frequency of the resonance ions.

The quantity k_(⊥)/ω_(ci) may be ˜v_(A), the Alfven velocity of the ions(cf. T. H. Stix, “Fast Wave Heating of a Two-Component Plasma,” NuclearFusion 15, 737 (1975)). Given the dependence of the Bessel function onν_(⊥), the coupling is proportional to powers of the perpendicularvelocity of the ions, and can be adjusted to preferentially damp on hottail ions from the neutral beam and on those diffused to higher energyby the radiofrequency waves.

Further, by setting the frequency of the radiofrequency generator 20according to the cyclotron frequency 35 at the turning point 34′, theinfluence of the electrical field from the radiofrequency generator 20on the plasma ions 36 is increased because of the prolonged dwell time50 of the plasma ions 36 at the turning point 34′ during their lowestaxial velocity as they turn around. This is in contrast, for example, tothermal plasma ions 32 which move quickly through this zone to furtherturning points 34 or which do not reach as far as the turning point 34′.

As noted above, by boosting the energy of the plasma ions 36 above thedistribution of thermal plasma ions 32 (per (b)) and by setting the RFgenerator 20 to an RF frequency which is a high harmonic of thecyclotron frequency 35 of the plasma ions 36, higher energy plasma ions36 having a higher radius of gyro-orbit 52 preferentially absorb powerover the thermal plasma ions 32 having a lower gyro-orbit 52. In someembodiments, the RF frequency may be set to a range from 20 to 100megahertz and/or to a harmonic n greater than n=2 and preferably n=4.

Generally, the higher harmonics boost the relationship between energyabsorption and gyro-orbit 52 according to increasing Bessel functionnumbers associated with those harmonics, Specifically, energy absorptionwill be proportional to J_(n-1)(k_(⊥)ρ) where: J_(n-1) is the Besselcoefficient for a given harmonic n, ρ is the radius of the particle'sgyro-orbit 52 about the magnetic flux lines 30 which increases withenergy by

$\rho = \frac{\left. \sqrt{}2 \right.{mE}}{eB}$

and k_(⊥) is a wave number of the plasma ions 36 being a property of thewave within the plasma and the polarization of the antenna 19 launchingthe wave.

It will be appreciated that this effective preferential absorption ofenergy by the plasma ions 36 will be self-reinforcing as energy isabsorbed and the gyro-orbit of the plasma ions 36 is increased.

Finally, by placing a highest field strength of the antenna 19 near theturning point 34′, the plasma ions 36 are preferentially influenced.

Generally, the magnetic containment field 15 will tend to lose someplasma ions 32 having low pitch angles through its ends. These particlesare said to be in the “loss cone.” By boosting the population of theplasma ions 36 having a known pitch angle θ outside of the loss cone,increased plasma densities can be obtained.

While the cyclotron frequency of the plasma ions 36 near the turningpoint 34′, and hence the desired setting of the frequency of theradiofrequency generator 20, is primarily a function of the vacuummagnetic field strength 37, it will shift slightly as a function ofincreasing plasma density/pressure. Accordingly, the inventioncontemplates that either or both of the DC power supply 18 or the RFfrequency generator 20 may be adjusted during operation to maintain theabove relationships which boost energy transfer to the plasma ions 36.In particular, this adjustment may be made via a closed-loop feedbackcontrol using a sensor 56 detecting plasma pressure, for example, usinga diamagnetic loop, which will measure the decrease in magnetic fielddue to increased plasma pressure to ensure a matching of the excitationfrequency of the RF generator 20 with the actual and dynamic cyclotronfrequency 35 at the turnaround point 34′. To the extent that thecyclotron frequency is dictated by the total field (vacuum field fromcoil plus plasma diamagnetism); the invention also contemplates that nofrequency change may be required but the location of the turning pointwill move closer to the electromagnetic mirror coil.

Referring now to FIG. 3 , this benefit of the present invention inproviding high plasma densities makes it useful as part of a systemwhere two high-energy plasma systems 10 may act as “plugs” to traphigh-energy plasma ions in a larger scale neutron generator 60 for thepurpose of transmutation (as discussed above) or fusion powergeneration. Such a design, for example, may make use of a tandem mirrorscheme, for example, described at G. Dimov, V, Zakaidakov, and M.Kishinevski, Fiz. Plazmy 2 597 (1976), [Sov. J. Plasma], Phys 2, 326(1976)] and T. K. Fowler and B. G. Logan, Comments on Plasma Physics andControlled Fusion 2, 167 (1977) and hereby incorporated by reference.

More specifically, in such a tandem mirror neutron generator 60, firstand second high-energy plasma systems 10 a and 10 b are placed inopposition along axis 14 flanking a generator volume 62, Generally, thehigh-energy plasma systems 10 will have an axial length on the order of2 meter whereas the generating volume 62 will be much larger, forexample, on the order of 50 meters or more.

The electromagnetic coils 16 of both of the high-energy plasma systems10 a and 10 b are axially aligned to provide a same direction ofpolarization of the magnetic field along the common axis 14. As such,the flux lines 30 of the first high-energy plasma system 10 a maycontinue through the volume 62 to the second high-energy plasma system10 b. Within the volume 62, the flux lines 30 are focused by anaxially-extending solenoid coil 66 circling the axis 14 around thevolume 62.

For this purpose, the electromagnetic coils 16 may be superconductingmagnets for example per D. Whyte, J. Minervini, B. LaBombard, E. Marmar,L. Bromberg, and M. Greenwald, “Smaller and sooner: Exploiting highmagnetic fields from new superconductors for a more attractive fusionenergy development path,” Journal of Fusion Energy, 35, 41 (2016) alsohereby incorporated by reference.

A subset of thermal plasma ions 32, having a uniform distribution ofpitch angles and having been boosted to higher energies by kinetictransfer from the plasma ions 36, may escape from the high-energy plasmasystems 10 into the volume 62 containing a reactant gas, for example,deuterium or tritium, to promote fusion and the emission of neutrons 64from the volume 62. The high pressure of the high-energy plasma systems10 blocks the escape of high-energy plasma ions from the volume 62 tomaintain the high densities for significant fusion.

The volume 62 may be surrounded by a contained volume 22 which mayinclude a heat exchanger liquid 68, for example, for receiving, throughone or more heat exchangers, a working fluid 70 of a thermodynamicengine such as a turbine or the like, for example, for the generation ofelectrical power. Alternatively, the contained volume 22 may be used forthe transmutation of materials to generate medical isotopes or torejuvenate spent nuclear fuel as discussed above.

The present application incorporates disclosure of US patent application2019/0326029 entitled: Apparatus and Method for Generating MedicalIsotopes, and US application 2013/0142296 entitled: Apparatus and methodfor generating medical isotopes which describe additional techniques formanaging isotope transmutation including the use of neutron multipliergenerators and other construction details and mechanisms for producing aneutral beam discussed above.

Certain terminology is used herein for purposes of reference only, andthus is not intended to be limiting. For example, terms such as “upper”,“lower”, “above”, and “below” refer to directions in the drawings towhich reference is made. Terms such as “front”, “back”, “rear”, “bottom”and “side”, describe the orientation of portions of the component withina consistent but arbitrary frame of reference which is made clear byreference to the text and the associated drawings describing thecomponent under discussion. Such terminology may include the wordsspecifically mentioned above, derivatives thereof, and words of similarimport. Similarly, the terms “first”, “second” and other such numericalterms referring to structures do not imply a sequence or order unlessclearly indicated by the context.

When introducing elements or features of the present disclosure and theexemplary embodiments, the articles “a”, “an”, “the” and “said” areintended to mean that there are one or more of such elements orfeatures. The terms “comprising” “including” and “having” are intendedto be inclusive and mean that there may be additional elements orfeatures other than those specifically noted. It is further to beunderstood that the method steps, processes, and operations describedherein are not to be construed as necessarily requiring theirperformance in the particular order discussed or illustrated, unlessspecifically identified as an order of performance. It is also to beunderstood that additional or alternative steps may be employed.

It is specifically intended that the present invention not be limited tothe embodiments and illustrations contained herein and the claims shouldbe understood to include modified forms of those embodiments includingportions of the embodiments and combinations of elements of differentembodiments as come within the scope of the following claims. All of thepublications described herein, including patents and non-patentpublications, are hereby incorporated herein by reference in theirentireties.

1. An apparatus for producing high-energy plasma comprising: a magnetic mirror containment field providing axially-extending magnetic flux lines converging at opposed first and second ends of a containment volume holding a plasma; a neutral beam generator directing a neutral beam of particles into the containment volume at an energy insufficient for fusion of the plasma ions; and a radiofrequency generator producing an electrical field to accelerate the plasma ions to an energy sufficient for fusion of the plasma ions.
 2. The apparatus of claim 1 wherein the frequency of the electrical field is functionally dependent on a cyclotron frequency at turning points for the plasma ions of the neutral beam in the magnetic mirror containment field.
 3. The apparatus of claim 2 wherein the frequency is a harmonic of the cyclotron frequency greater than the cyclotron frequency.
 4. The apparatus of claim 1 wherein the energy of the neutral beam is set so that more than 50 percent of the neutral beam particles are converted to plasma ions.
 5. The apparatus of claim 1 wherein the neutral beam has an energy of less than 50,000 electron volts.
 6. The apparatus of claim 1 wherein the radiofrequency generator boosts the energy of the plasma ions from the neutral beam by more than 2 times.
 7. The apparatus of claim 1 wherein the radiofrequency generator includes an antenna positioned to be proximate to a reflection limit of the plasma ions and to generate a rotating electric vector perpendicular to the axis of the magnetic mirror containment field.
 8. The apparatus of claim 7 wherein the pitch is between 30° and 60° to the axis.
 9. The apparatus of claim 1 further including a treatment volume at least partially surrounding the containment volume to receive high-energy neutrons therethrough and containing an element for transmutation into a different element.
 10. The apparatus of claim 9 wherein the element for transmutation is a precursor to a medical radioisotope selected from the group consisting of precursors ⁹⁹Mo, ¹³¹I, ¹³³Xe; and ¹⁷⁷Lu.
 11. The apparatus of claim 9 wherein the element for transmutation is an exhausted nuclear fuel.
 12. The apparatus of claim 1 wherein the neutral beam is selected from the group consisting of deuterium and tritium.
 13. The apparatus of claim 1 wherein the neutral beam is deuterium.
 14. The apparatus of claim 1 wherein including a pair of magnetic coils producing the magnetic mirror containment field and wherein the radiofrequency generator provides an antenna between the magnetic coils and wherein the containment volume is contained within a gas-tight chamber.
 15. A fusion apparatus comprising: a reaction volume holding a fusible material within a first axially-extending magnetic containment field; a first and second plasma plug flanking the reaction volume along the axis; each plasma plug including: (a) a magnetic mirror containment field providing axially-extending magnetic flux lines converging at opposed first and second ends of a containment volume holding a plasma; (b) a neutral beam generator directing a neutral beam of particles into the containment volume at a predetermined pitch and energy so that the particles disassociate into plasma ions within the containment volume; and (c) a radiofrequency generator producing an electrical field to accelerate the plasma ions to an energy above that of the particles entering the containment volume; whereby plasma ions escaping from the first and second plasma plug produce a fusion reaction in the reaction volume.
 16. The fusion apparatus of claim 15 wherein a pressure of plasma ions within the first and second plasma plug is greater than a pressure of plasma ions within the reaction volume.
 17. The fusion apparatus of claim 16 further including an electrical generator receiving neutrons from the reaction volume to generate electrical power. 